It uses an appliance already employed for this purpose and in normal use notably in the nuclear industry: it is referred to as a bubble elevator and consists of a generally vertical tube, open at its ends, which also comprises an air-inlet tapping at an intermediate height. The bottom end is always immersed in the liquid in the tank and corresponds to a liquid entry opening, whilst the top end can be emerging or immersed according to circumstances and corresponds to an opening for ejecting a mixture of air and liquid: this is because the air introduced into the tube through the tapping rises, entraining with it the liquid occupying the tube, until it emerges through the top end, and the ejected liquid is replaced by liquid from the tank entering through the bottom opening. Bubble elevators do not require the use of moving mechanical parts and can therefore be employed even in corrosive liquids; they are very effective for stirring and homogenising the liquid in the tanks, notably in order to put back in suspension any deposits which have accumulated at the bottom of the tank.
The efficiency of a bubble elevator can be defined as the flow of entrained liquid divided by the flow of air supplied. It is found that it is very variable and depends greatly on the degree of immersion, that is to say on the ratio between the height separating the air tapping from the level of liquid in the tank and the height, which is invariable, between the air tapping and the top of the elevator; the first of these heights corresponds in reality to the difference in level between the air tapping and the free surface of the liquid. If the liquid is flush with the top of the elevator, the latter is entirely immersed and the degree of immersion is 100%; this degree decreases as the liquid drops in the tank. It is found that the efficiency drops rapidly with the degree of immersion and that it becomes zero for a degree of immersion equal to approximately 1/3, which means that the liquid no longer emerges at all from the top opening of the elevator. It must therefore be accepted that bubble elevators become inoperative for tanks with a low level of liquid. When the level in the tank is variable, several elevators of different heights are arranged and used separately according to circumstances, choosing the one which for the time being will have the best efficiency or the one which will open out at depths which will afford the most complete homogenisation. However, it is correct to judge that the installation becomes complicated.